Spark plug with main and teaser gaps in parallel



Jan. 6, 1970 H. A. BURLEY 3,488,556,

SPARK PLUG WITH MAIN AND TEASER GAPS [N PARALLEL Filed Dec. 1, 1967 l IZOI ONLY BOTH TRIGGERGAP PRESSURE 9O TRIGCFBER AND MAIN GAP FIRE I PSIG AIR 1 24' 6O CONVENTIONAL I 0.0IOIH.TRIGGER GAP 2 SPARK PLUG .O35m.GAP 7 omsmMAlN GAP Is- I O lz sllfilglylslg s 0 I0 I0 10 I0 Io Io I0 10 I0 I TRIGGER GAP R N E W OHMS IN KY I27 I SPARK PLUG TRIGGE RESISTA c .35In.MAIN GAP {1 I 8- o IIITRIGGER GAP RY=IO5I1 INVENTCIR Harvey/Z gar/I? PSIG AER United States Patent 3,488,556 SPARK PLUG WITH MAIN AND TEASER GAPS IN PARALLEL Harvey A. Burley, Warren, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed Dec. 1, 1967, Ser. No. 687,203 Int. Cl. H01 7/44; H01k 1 62 US. Cl. 315-60 9 Claims ABSTRACT OF THE DISCLOSURE An improved spark plug having a ground electrode, center electrode and a third electrode is disclosed in which a resistor is connected in serie with the third electrode. Voltage impressed upon the spark plug initiates arcing across the small gap between the center electrode and third electrode which continues for a time sufficient to reduce the voltage required to initiate sparking across the main gap between the center electrode and the ground electrode.

This invention relates to spark plugs, and more particularly to three electrode spark plugs.

Present day automobile spark plugs employ an ignition system which delivers up to 32 kv. to effect the sparking across the gap between the center electrode and the ground electrode. The voltage required to cause a spark to jump across the gap is referred to as the breakdown voltage and is about 7 to 24 kv. under normal operating conditions. Electrical insulation problems are encountered when a high breakdown voltage such as this is used. Another problem resulting from this high breakdown voltage is the amount of radio frequency interference which is generated thereby.

It is an object of this invention to provide an improved spark plug for reciprocating engines. It is another object of this invention to provide a spark plug which is operative on a relatively low voltage. It is still another object of this invention to provide a spark plug having relatively low radio frequency interference.

These and other objects are accomplished by a three electrode spark plug having two spark gaps. In additon to the conventional main park plug gap between the center electrode and the ground electrode, a small or trigger gap is formed between the center electrode and the third electrode. A resistor is connected in series with the third electrode so that voltage impressed upon the spark plug causes arcing across the trigger gap which continues for a time sufficient to ionize the gaseous molecules in the main gap region thereby reducing the breakdown voltage of the main gap, that is, the voltage-required to initiate sparking across the main gap between the center electrode and the ground electrode. A voltage of about kv. is required at a pressure of 120 pounds per square inch with this three electrode spark plug compared to a voltage of 18 to 20 kv. with a conventional spark plug at the same pressure. This spark plug has reduced electrical insulation problems and substantially reduced radio frequency interference.

Other objects and advantages of this invention will be apparent from the following detailed description, reference being made to the accompanying drawings wherein preferred embodiments of this invention are shown.

In the drawings:

FIGURE 1 is a side view of an improved three electrode spark plug;

FIGURE 2 is an enlarged view of the firing tip portion of a second embodiment of an improved three electrode spark plug;

FIGURE 3 is an enlarged view of the firing tip portion 3,488,556 Patented Jan. 6, 1970 of a third embodiment of an improved three electrode spark plug;

FIGURE 4 indicates the effect of the resistor size on the spark plug breakdown mode; and

FIGURE 5 indicates the difference in breakdown voltage between a conventional spark plug and the improved three electrode spark plug.

The use of three electrode spark plugs having two sparking gaps is old in the art as evidenced by the patent to Radtke, No. 1,290,780 and the patent to Springer 1,399,166. Attempts to utilize a third electrode to reduce the voltage required for sparking across the main gap have heretofore had limited success primarily because the sparking in the second gap wa not of sufiicient duration or intensity to have an effect on the main gap region.

Referring now to the drawings, in FIGURE 1 of the spark plug 10 comprises a conventional outer metal shell 12 having a ground electrode 14 Welded to the lower end thereof. Positioned within the metal shell 12 and secured in the conventional manner is the insulator 16. The insulator 16 should preferably be of a high alumina base material containing upwards of aluminum oxide such, for example, as covered by United States Patent No. 2,760,877, issued to Karl Schwartzwalder and Helen Blair Barlett. The insulator 16 is formed with a stepped centerbore 18 having a shoulder 19.

Positioned within the insulator ceuterbore 18 is a tubular third electrode 20 having a flared upper portion 22 supported by the insulator shoulder 19. Positioned within the tubular third electrode 20 is a tubular insulator 24 having an upper flared end portion 26. The insulator 24 should preferably be of a high alumina base material containing upwards of 85% aluminum oxide of the type described previously for the insulator 16. Positioned within the tubular insulator 24 is the center electrode 28. The insulator 24 electrically insulates the center electrode 28 from the third electrode 20. A glass mass 30 hermetically seals the center electrode 28 to the insulator flared portion 26, the third electrode flared end 22. and the insulator 16. The lower end 32 of the tubular insulator 24 is preferably flush with the center electrode firing tip end 34 and the third electrode firing tip end 36.

The center electrode firing tip end 34 and the third electrode firing tip end 36 form a small or trigger gap therebetween. The length of the trigger gap existing between the third electrode and the center electrode is from about 0.001 inch to 0.020 inch with the preferred length being 0.003 to 0.010 inch. The length of the trigger gap is usually selected on the basis of performance under adverse conditions such as fouling, flood start, and cold start.

A resistor 38 is located in insulator 16 and connected in series between the outer shell 12 and. the tubular third electrode 20. While the resistor may he in the form of a unitary body which is connected by leads to the shell and the center electrode, the resistor may take the form of a solid body which in itself makes electrical contact with the third electrode 20 and the shell 12. As shown in FIGURE 2, the resistor 38A is a solid mass passing through a hole or opening 39 in the insulator 16 and is in electrical contact with the third electrode and the shell. As shown in FIGURE 3, the resistor may take form of an annular button 38B having a centerbore 41 which encircles and is in contact with the tubular third electrode 20 and an outer surface which is in contact with the shell. In addition to the various positions Within the spark plug 10, as shown in FIGURES 1, 2 and 3, the resistor 38 may be positioned outside of the spark plug 10 as long as the resistor is connected in series with the third electrode 20 and is also connected to a ground. The position of the resistor is not critical in the practice of this invention.

The resistor 38 should have a resistance in the order of 10 ohms to be Operative under the pressure conditions most commonly encountered in automotive application. As can be seen in FIGURE 4, if the resistance of the resistor 38 is too low, only the trigger gap fires whereas resistances above ohms will operate both the trigger gap and the main gap under all conditions including the extreme high pressure conditions of about 120 p.s.i.

The center electrode firing tip portion and the firing tip end portion of ground electrode 14 form the conventional spark plug main gap. The main gap is about 0.035 inch in length. The length of the main gap depends upon the particular engine in which it is used. As it is well known in the art, a small gap is more efficient for igniting good fuelair mixtures whereas a large gap is more efiicient for igniting lean mixtures and rich mixtures.

The three electrode spark plug described above is connected in an ignition system which is a modification of the conventional inductive ignition system. The conventional inductive system containing a coil having a 100 to 1 turn ratio and delivering up to 28 kv. is modified by using a coil having a 30 to 1 turn ratio which in turn delivers about 8 kv.

The ignition system delivers the voltage to the spark plug initiating arcing across the small or trigger gap which continues for a time sufficient to ionize some of the gas in the main gap. The ionization of the gas in the main gap reduces the voltage required to cause a spark to jump across the main gap. In conventional spark plugs it is necessary to deliver up to 24 kv. depending upon the pressure to cause a spark to jump across the main gap. The three electrode spark plug of this invention will, due to the ionization of gas in the main gap caused by the arcing in the trigger gap, spark across the main gap with a voltage ranging from 2 to 6 or 7 kv. depending upon the operating conditions and the pressure in the combustion chamber.

In one of the spark plugs described above, when the current passing through the spark plug under simulated engine conditions, the breakdown voltage required for a spark to jump across the 0.008 inch trigger gap was found to be 8 kv. When the spark first jumped across the trigger gap the voltage dropped momentarily to 3 kv. and then started to rise with the trigger current continuing to flow. When the voltage reached 5 kv. microseconds later, sparking across the 0.035 inch main gap commenced. Under heavy load conditions, that is pressure in the range of 100 to 120 p.s.i., even the trigger spark by itself is sufiicient to ignite the fuel-air mixture.

The presence of a trigger gap lowers the breakdown voltage of the main gap significantly depending upon the pressure at which the sparking occurs. As shown in FIG- URE 5, a conventional spark plug having a main gap of 0.035 inch has a breakdown voltage at 120 p.s.i. of about 20 kv. whereas the improved three electrode spark plug having the same size main gap and a trigger gap of 0.010 inch is 5 kv. The breakdown voltage of the trigger gap spark plug is affected to a lesser degree by the pressure than the conventional spark plug in respect to the breakdown voltage.

The three electrode spark plug of this invention permits the use of a lower voltage in obtaining sparking across the main gap. Since the breakdown voltage is considerably less than in the conventional spark plug, the radio frequency interference is significantly reduced.

While the invention has been described in terms of a preferred embodiment, it is to be understood that the scope of the invention is not limited thereby except as defined in the following claims.

I claim:

1. A spark plug comprising a metal shell having a ground electrode secured to one end thereof, an insulator having a centerbore therethrough secured within said shell, a center electrode positioned in one end of said centerbore and having an end portion extending through said insulator into spaced relationship with said ground electrode to form a first spark gap therebetween, a third electrode positioned in spaced relationship with said center electrode and having an end portion adapted to form a second spark gap with said center electrode and a resistor connected in series with said third electrode wherein a voltage impressed upon said spark plug initiates arcing in said second gap which continues for a time sufiicient to ionize gas in said first gap thereby reducing the voltage required to initiate sparking in said first gap.

2. A spark plug as described in claim 1 wherein said second gap is 0.001 inch to 0.020 inch.

3. A spark plug as described in claim 1 wherein said resistor has a resistance of at least 10 ohms.

4. A spark plug as described in claim 1 wherein said resistor is connected in series between said shell and said third electrode.

5. A spark plug as described in claim 1 wherein said resistor is connected in series between said third electrode and a ground.

6. A spark plug comprising a metal shell having a ground electrode secured to one end thereof, a first insulator having a centerbore therethrough secured within said shell, a tubular third electrode positioned within said first insulator centerbore, a second tubular insulator having a centerbore therethrough positioned in said tubular third electrode, a center electrode positioned in said tubular insulator centerbore and having an end portion extending through said first insulator into spaced relationship with said ground electrode to form a first spark gap therebetween, said third electrode having an end portion adapted to form a second spark gap with said center electrode and a resistor connected in sereis with said third electrode wherein a voltage impressed upon said spark plug initiates arcing in said second gap which continues for a time sufiicient to ionize gas in said first gap thereby reducing the voltage required to initiate sparking in said first gap.

7. A spark plug described in claim 6 wherein said center electrode firing tip end and said tubular insulator end and said third electrode firing tip end form a flush surface where they meet.

8. A spark plug comprising a metal shell having a ground electrode secured to one end thereof, an insulator having a centerbore therethrough secured within said shell, a center electrode positioned in one end of said centerbore and having an end portion extending through said insulator into spaced relationship with said ground electrode to form a first gap therebetween, a third electrode positioned in spaced relationship with said center electrode and having an end portion adapted to form a second gap with said center electrode and a resistor body in electrical contact with said third electrode and said metal shell wherein a voltage impressed upon said spark plug initiates arcing in said second gap which continues for a time sufficient to ionize gases in said first gap thereby reducing the voltage required to initiate sparking in said first gap.

9. A spark plug as described in claim 8 wherein said resistor body is in the form of an annular button.

References Cited UNITED STATES PATENTS 1,290,780 1/1919 Radtke 313- X 1,399,166 12/1921 Springer 3 l3140 X 2,963,620 12/1960 Knudson et al 313-131 X 3,146,301 8/1964 Logan 313-131 X 3,344,304 9/ 1967 Rademacher 313131 3,049,644 8/1962 Bowlus et al. 3l560 X JAMES W. LAWRENCE, Primary Examiner C. R. CAMPBELL, Assistant Examiner Us. (:1. X.R. 513* 131. 140 

